1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses suited to video cameras.
2. Description of the Prior Art
Since the image pickup elements of the video cameras generally have a low sensitivity, the photographic lenses for use therewith are desired to be as fast as possible. It is also desired to shorten the photographic lens in the longitudinal direction for the purpose of achieving a reduction of the bulk and size of the video camera. These desires relate to the fact that though the image pickup tubes used in the video cameras have generally been of 2/3 in. size, from the standpoints of compactness and image quality, it is in recent years that people, seeking a further improvement in manageability and reduction of price, have sought the realization of a so-called 8 m/m video camera the size and weight of which are on the same order as those of the 8 m/m cine camera. Along with this, the image pickup tube itself must be reduced in size. For this reason, at least for present purpose the 1/2 in. tube is employed.
For the 2/3 in. tube, to remedy the low sensitivity of the tube, the lens had to be as fast as about F/1.4. For the 1/2 in. tube, however, because its image receiving area is equal to 1/2 times that of the 2/3 in. tube, the speed of the lens is necessarily at least F/1.2. To realise a zoom lens of as high a relative aperature as F/1.2, it is required to better correct the residual aberrations of the varifocal section than was heretofore possible. For this reason, how to design the image forming lens section becomes more important than ever. The use of known lens design techniques generally results in making it more difficult to correct spherical aberration in the central zone of the image field, and comatic aberrations and sagittal halo in the intermediate to the marginal zone as the relative aperture increases. Accordingly, there was a drawback in that the contrast was caused to be lowered. This tendency became prominent particularly when a great advance in the compactness of the lens system was aimed at.